I always use Castrol GTX in every car I own. It to me is like the difference between the ice cream in the big plastic tubs, and the ice cream in the little tiny containers that say's Godiva, or Ben and Jerry's, etc.
I have used Castrol GTX in my small engine Karmann Ghia, air cooled for 15 years, never had an engine problem yet. My '79 diesel has had Castrol since I got it, and the engine runs and sounds better than when I first started driving it. The reason I use Castrol in my diesel is because my friend has a '79 diesel 300SD, and an '80 450 SEL both purchase new and owned and run on Castrol for these 20 years. The diesel is way over 400,000 miles and still going strong.
I have switched to other brands out of necessity before, and I found I wanted to change the oil back to Castrol after only a few hundred miles. The valves sound louder, the oil seems less "oily", and more watery, especially Pennzoil. Their color, and weight seems like water to me. It makes me very uneasy.
'79 300SD 345,000 miles
'77 FJ40 Land Cruiser 130,000 miles
'71 Karmann Ghia convertible 3-4-or 500,000 miles, I've lost track of how many times it's gone around.